1. Field of the Invention
The present invention relates to a novel method of arc melting and casting highly active (highly reactive with oxygen) metals, such as titanium, titanium alloy, niobium, tantalum, nickel, chrome and cobalt, and a novel apparatus thereof.
2. Prior Art
As well known, various metals are molten and cast using arc. With this conventional well-known melting and casting method, a metal material on a crucible in an airtight chamber is molten by an arc column generated from an arc generation electrode disposed in the airtight chamber and is poured into a casting mold disposed to receive the molten metal from the crucible.
With this method, however, the metal material begins to melt at the arc generation point and the molten area is gradually widened by heat conduction. Since melting proceeds in this way, the arc column is locally concentrated at the arc start point. This phenomenon is noticeable especially when the heat conductivity of the material (such as cobalt, nickel, chrome and titanium) is low. As a result, the material is locally overheated and casting defects such as rough casting surface and cavities are apt to generate. When an alloy (for example Ti-6Al-4V) including a metal which is easily lost due to evaporation (for example aluminum) is cast, the metal evaporates due to overheat and the composition of the alloy may change.
In addition, scraps or casting materials having irregualr shapes cannot be directly used since the arc column is fixed at one point. These materials must be molded into a regular shape (for example a cylindrical block) using a molding crucible before they are molten and cast. This requries extra electric power, processes and a molding crucible. Furthermore, since the contact area between the molten metal and the crucible is wide, the heat loss of the molten metal to the crucible is great and cannot be reduced by providing heat insulation grooves in the crucible. As a result, great electric power is consumed.
Moreover, in the case of a casting method wherein molten metal is poured through a hole in the bottom of the crucible into a casting mold closely contacting the crucible, the molten metal does not drop promptly but drops intermittently. This is apt to cause casting defects such as misrun and cold shuts. These are the problems pertinent to the conventional method.